Directional drilling tools proposed hitherto have a sleeve member disposed on the drillstring adjacent the drillbit and provided with laterally extendable and retractable steering elements operable selectively by means of pressure of the drilling mud to press on the hole wall or borehole and so steer the drillbit. Typical of such tools are those described in, for example, U.S. Pat. Nos. 2,316,409 (Downing) and 3,595,326 (Claycomb); and French Patent Publication 2622920 (SMF International). One disadvantage of these previously proposed tools is that they have not been immediately responsive to actioning of the steering elements due to a delay while these elements move across the well annulus. An associated disadvantage is that a significant portion of the available range of steering element movement, ie. the cross-annulus movement, is not useful for steering.
An object of the present invention is to provide apparatus for directional drilling whereby the mentioned disadvantages may be obviated or mitigated.